Oscillator having reduced sensitivity to acceleration

ABSTRACT

An oscillator includes resonator portions each mounted to a substrate via one or more mountings at one end and having an active resonance region defined between a set of electrodes. Each resonator portion has a longitudinal axis directed along the resonator portion from the mounted end to a free end. The resonator portions are mounted such that their longitudinal axes are directed in different directions, e.g. an anti-parallel arrangement, so that acceleration sensitivity vector components aligned with the longitudinal axis cancel each other out.

FIELD OF THE INVENTION

The present invention relates to an oscillator having reducedsensitivity to acceleration. In particular, it relates to an oscillatorcomprising two asymmetrically mounted resonators.

BACKGROUND OF THE INVENTION

Electronic devices often include an oscillator to provide an oscillatingsignal for use as a clock source. The oscillating signal can becontrolled by a resonator which requires some form of excitation signalto sustain oscillations.

The operation of an oscillator, controlled by the resonator, may beaffected by age and certain environmental conditions such as temperatureand, of particular interest here, acceleration. When an oscillator issubjected to acceleration, the frequency of the oscillating signal itproduces may be altered. The change in frequency is proportional to themagnitude of the acceleration and dependent on direction, giving rise toan acceleration sensitivity vector. A time variable acceleration, forexample vibration, can cause frequency modulation of the oscillator'sfrequency. Reducing the sensitivity of such oscillators to alterationsof frequency due to acceleration is therefore desirable in order toproduce a stable and pure frequency output from an oscillator.

Known techniques for reducing the sensitivity of oscillators toacceleration may involve packaging and/or resonator design andcompensation techniques.

Considering first packaging, this typically involves configuring thepackaging so as to achieve a symmetrical crystal resonator design. Ithas been demonstrated to a first approximation that a theoretical zeroexists for the acceleration sensitivity when considering a perfectlysymmetric bulk acoustic wave (BAW) resonator and mounting structure.However, achieving a symmetric mounting structure which maintainssymmetric stress on the resonator can be a difficult task due tomanufacturing imperfections. The configuration of the structure alsoimposes further problems in terms of stress on the resonator. Springclips may typically be employed to mount the resonator, so as to reducestress applied from the surrounding enclosure, but performance may bedegraded by resonance of the spring clips themselves, which in turn maylead to amplification of any applied acceleration. Such techniques thusinvolve relatively large, elaborate designs and complex manufacturingmethods.

Turning now to compensation techniques, these mainly fall into threecategories: i) mechanical, ii) active and iii) passive. The firstcategory of compensation techniques includes mechanical vibrationisolation or dampening techniques. These tend to only affect the upperportion of the vibration frequencies associated with standardconditions, i.e. the upper portion of the ˜0 to 3 kHz frequency range.Compensating in this manner in the lower portion of the ˜0 to 3 kHzfrequency range becomes increasingly difficult due to the necessaryincrease in size of the mechanical isolation system required to filterout the larger amplitudes. Moreover, the vibration isolation system isitself a resonant structure, which can amplify the vibration at andbelow its resonant frequency and is often only effective along onedirection.

The second category of compensation techniques involves passive deviceswhere multiple resonators are located to form a composite oscillator insuch a way as to cancel out the effects of acceleration. This isdemonstrated for example in U.S. Pat. No. 4,410,822 and also in U.S.Pat. No. 4,575,690 where individual crystal resonators are manufacturedand then analysed in order to ascertain their acceleration sensitivityvectors. Once their acceleration sensitivity vectors are known, matchingresonators can be selected and mounted in an anti-parallel arrangementin an attempt to try and cancel out the respective accelerationsensitivity vectors of the resonators. Such techniques suffer from theimpracticalities of having to measure, select and precisely mount theresonators.

The third category of compensation techniques involves so-called activedevices, where one or more accelerometers are used to sense the appliedacceleration. The applied acceleration signal can then be used tocontrol a frequency modulation circuit to cancel out the frequencychanges induced in a resonator from the applied acceleration. Thisapproach suffers from complications concerned with matching the responseof the accelerometer to the resonator over a wide range of vibrationfrequencies, obtaining accurate accelerometer alignment, and alsoapplying the correction signal to the oscillator frequency in a linearmanner.

To overcome the impracticalities encountered with the above compensationtechniques, multi-oscillator arrangements have been proposed, forexample the arrangement described in U.S. Pat. No. 5,250,871, where fouror more resonators are electrically connected in series, with theiracceleration sensitivities aligned such that at there are at least twopairs of opposing vectors in each plane. Effectively the “law of largenumbers” is used to overcome unit-to-unit variations, so that the moreunits, the more likely it is that their acceleration sensitivity vectorswill cancel each other out. Such techniques are inherently costly, bothfinancially due to the number of units, and in terms of space requiredfor the multi-oscillator arrangements.

FIG. 1 shows side 2 and plan 4 views of a known resonator and package.The oscillator contains a strip resonator 12 mounted to a substrate 16by a pair of mountings 20. The mountings 20 provide mechanical supportand allow for electrical coupling of the resonator. The electricalconnections may be in the form of excitation electrodes located above 18a, and below 18 b, the resonator to sustain its oscillations. Theresonator 12 may for example include a piezoelectric material such asquartz crystal. The resonator 12 is enclosed within a housing 22 forproviding mechanical and environmental protection for the resonator. Theform, material and method of adhering the housing components to eachother and to the substrate is well known in the art, and includeshermetic packaging which may consist of sealing via a Kovar seal ringand seam welded lid. The substrate may for example be made of a ceramicmaterial.

Each resonator has an active resonance region, which is the area definedby electrodes 18 a and 18 b. It is desirable that this area should befree from any mechanical restriction in order to allow the resonator tofunction correctly. As such it is desirable to isolate the resonatorsfrom as much mechanical stress as possible. To help achieve this, theresonator can be mounted in a cantilever fashion, with the activeresonance region remote from the mounting region. These structures arerobust, provide good decoupling of mounting strain, and are relativelystraightforward to assemble. The cantilevers may be mounted usingelements that naturally aid the relaxation of the stress between thepackage and the resonator, for example using compliant conductiveadhesive. A limitation with such resonators is that the magnitude of theacceleration sensitivity vector along the length of the resonator can berelatively high. So, whilst cantilever-mounted resonators may be suitedto situations in which compactness and flatness requirements areparamount, and in which relatively high alterations of resonantfrequency due to acceleration can be tolerated, a cantilever-mountedresonator is far from ideal in applications requiring low sensitivity toacceleration.

FIG. 1A shows a perspective view of a strip resonator 200 with mountings206. An active resonance region is defined by electrodes 202 located oneither side of the resonator (connections not shown). The orthogonal x,y and z axes of the resonator are shown by coordinate system 204.

FIG. 1B shows a side view of resonator 200 with upper and lowerelectrodes 202 and mountings 206. When the resonator is subjected toacceleration in the z direction, i.e. along the z axis 208, stress isapplied to the resonator through the mountings 206. The stress appliedto the resonator varies across the profile of the resonator in the zdirection, leading to a so-called non-uniform stress distribution, whichis most acute in the region around the mountings 206. The non-uniformityof the applied stress due to acceleration decreases approximatelyexponentially with distance along the length of the cantilever away fromthe mountings (in the y direction). This means that the non-uniformityof the applied stress in the z direction decreases approximatelyexponentially with distance from the mountings to the free end of theresonator, i.e. with distance along the y axis, shown by graph 214.Therefore at a position 210 relatively close to the mountings, thestress applied to the resonator will be relatively non-uniform in the zdirection, as indicated by arrows of differing lengths. At a position212 relatively far from the mountings, the stress applied to theresonator will be relatively uniform in the z direction, as indicated byarrows having the same length. This effect is particularly pronounced inrelation to small dimensions of resonator, which in this case is the zdimension (this being very small compared to the y and x dimensions ofthe resonator).

A similar effect occurs when the resonator is subjected to accelerationin the x direction. FIG. 1C shows a plan view of resonator 200,electrodes 202 and mountings 206. When the resonator is subjected toacceleration in the x direction, i.e. along the x axis 220, stress isapplied to the resonator through the mountings 206. The non-uniformityof the applied stress in the x direction decreases approximatelyexponentially with distance from the mountings, i.e. with distance alongthe y axis, shown by graph 226. Therefore at a position 222 relativelyclose to the mountings, the stress applied to the resonator will berelatively non-uniform in the x direction, as shown by arrows ofdiffering lengths. At a position 224 relatively far from the mountings,the stress applied to the resonator will be relatively uniform in the xdirection, as indicated by arrows having the same length.

The stress applied uniformly along the edge of the resonator active areain the x and z directions leads to equal and balanced areas of tensionand compression. These tend to cancel out any induced resonant frequencychange and so lead to a low acceleration sensitivity.

The situation is different when the resonator is subjected toacceleration in the y direction as the y direction is aligned with thedirection between the mountings and the resonator active area instead ofbeing orthogonal thereto, as in the case for the x and z directions.FIG. 1D shows a plan view of resonator 200 with electrodes 202 andmountings 206. When accelerated in the y direction by a force appliedfrom the end of the resonator where the mountings 206 are located, thenon-uniformity of the applied stress in the y direction does notdecrease exponentially with distance from the mountings along the ydirection. In this case, the whole of the resonator is in compression,the compressive strain ranging from a maximum at the mountings to aminimum at the free end of the resonator. Similarly, when accelerated inthe y direction by a force applied from the free end of the resonator,the whole of the resonator is in tension, the tension strain rangingfrom a maximum at the mountings to a minimum at the free end of theresonator. As a result there is a non-uniform and unbalanced stressdistribution in the y direction across the active resonance region, asindicated by arrows 230 of differing lengths, and either the whole ofthe active region is in compression, or the whole of the active regionis in tension.

In either of these cases, the applied stress changes from beingnon-uniform in the x and z directions at a position 240 relatively closeto the mountings to being relatively uniform in the x and z directionsat a position 242 relatively far from the mountings, where the activeresonator region is located. In the y direction though, the appliedstress is not uniform across the active resonance region of theresonator. In fact, the applied stress changes approximately linearlywith distance along the resonator due to the linear change in resonatormass remaining to be accelerated, as indicated by arrows 230 ofdiffering lengths.

This imbalance between areas of tension and compression over the activeresonator region leads to an induced resonant frequency shift and hencea relatively high acceleration sensitivity in this y direction.

It would thus be desirable to implement a low-cost solution that reducesthe acceleration sensitivity of an oscillator without the need formeasurement, selection and precise specific individual alignment ofresonators.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided an oscillator comprising:

a plurality of resonator portions, each said resonator portion having anactive resonance region defined between a set of electrodes capable ofreceiving signals caused by oscillator resonance in said activeresonance region during operation of said oscillator,

each of said resonator portions being mounted in said oscillator usingone or more mountings, each resonator portion comprising first andsecond ends, the first end of each resonator portion being mounted to asubstrate via at least one of said one or more mountings, the second endof each resonator portion being free, the resonator portions each havinga respective longitudinal axis which is directed along the resonatorportion from the first end to the second end of the resonator portion,

wherein said resonator portions are mounted in said oscillator such thatthe longitudinal axes of the resonator portions are directed indifferent directions so as to reduce the sensitivity of the oscillatorto acceleration.

In this manner, components of acceleration sensitivity directed alongthe longitudinal axes may be cancelled.

In an embodiment of the invention, an oscillator comprises a pluralityof resonator portions, each said resonator portion having an activeresonance region defined between a set of electrodes capable ofreceiving signals caused by oscillator resonance in said activeresonance region during operation of said oscillator, each of saidresonator portions being mounted asymmetrically in said oscillator usingone or more mountings, the asymmetry of said mountings with respect toeach active resonance region defining, for each active resonance region,a dominant axis passing through the active resonance region andproviding a relatively large acceleration sensitivity along saiddominant axis and relatively small acceleration sensitivities along axesorthogonal to said dominant axis, said resonator portions being mountedin said oscillator such that the acceleration sensitivities, of therespective active resonance regions along their respective dominantaxes, are directed in different directions so as to reduce thesensitivity of the oscillator to acceleration.

Embodiments of the present invention thus propose an oscillator withasymmetric mounting of a plurality of resonator portions each having anactive resonance region. The asymmetric mounting of the resonatorportions means that each resonator portion has an axis passing throughits active resonance region along which the acceleration sensitivityvector is dominant, i.e. the sensitivity to acceleration along thedirection defined by the dominant axis is much greater than thesensitivity to acceleration in other directions. As described in thebackground section, this is because, when an individual resonator issubjected to acceleration in the dominant y axis direction, there is nobalancing of applied stress across the active resonance region in the ydirection, as either the whole of the active resonance region is incompression or the whole of the active resonance region is in tension,whereas when the resonator is subjected to acceleration in either the xor z directions, an expansion in one part of the resonator is balancedby a contraction in another part of the resonator giving rise to abalanced strain distribution across the resonator in these directions.

As also described above in the background section, a dominantacceleration sensitivity vector has made such asymmetrical resonatorsundesirable in some applications. However, the present invention usesthis to its advantage, and by mounting the resonators in a compositeoscillator such that their dominant axes are directed in differentdirections, e.g. an anti-parallel arrangement, the dominant accelerationsensitivity vectors can cancel each other out. The components of theacceleration sensitivity along the non-dominant axes orthogonal to thedominant acceleration sensitivity vector are still present, but as theireffect on the total acceleration sensitivity of the composite oscillatoris much less, the result is a composite oscillator with reducedsensitivity to acceleration.

It will be appreciated from the foregoing that the accelerationsensitivity along the non-dominant axes can be reduced if balancing ofstrain occurs. For this to occur, the stress applied by the accelerationmust be applied uniformly across the active resonance region of theresonator. Therefore, preferably, the active resonance region is locatedin a region where the applied stress is uniform in both the non dominantx and z axis directions.

Preferably there are two resonator portions and, as mentioned above,their dominant acceleration sensitivity vectors are directed in oppositedirections, i.e. anti-parallel, such that they cancel each other out.Because of the asymmetrical mounting of the resonators, the dominantsensitivity axes of the resonators can be predicted without measurementor precise selection of the resonators, which can be costly and timeconsuming processes. Further, no individually specific alignment isneeded to direct the dominant axes of the respective resonators duringmounting of the resonators in the oscillator.

In one arrangement, the oscillator may include two or more separatelymounted individual resonator portions, which may be mounted within aseparate housing or within the same housing.

In an alternative arrangement, the resonator portions may be integrallyformed and may be connected by a central portion via which the resonatorportions are mounted. This arrangement has the added advantage that,since the resonator portions are part of an integral structure, thehousing for the portions may be smaller; in addition, coordination oftheir respective orientations is not required in order to locate the tworesonators in an anti-parallel manner, since by definition theorientations are automatically aligned.

Preferably, the resonator portions are mounted to a substrate by acantilever mounting arrangement such that a first end of each resonatorportion is mounted and a second end is free. The cantilever mountingarrangement may include a pair of mountings adjacent the first end ofeach resonator portion and provide mechanical coupling to the substrateand also electrical coupling to the electrodes.

Preferably, each active resonance region is located towards the second,free end of each resonator portion. As explained above, the amount ofnon-uniform stress in a resonator portion decreases with distance fromthe resonator portion mounting point(s). Thus locating the activeresonance region away from the mountings means that the applied stressin the active resonance region along the non-dominant axes is moreuniform than it would have been had the active resonance region beenlocated in the vicinity of the mountings. When the active resonanceregion is positioned far from the mounting points, the accelerationsensitivity along the non-dominant axes is therefore reduced. Moreover,since the stress distribution in the dominant axis direction is notuniform in regions where the stress distribution in the non-dominantaxes is uniform, locating the electrodes in these regions increases theasymmetry of the resonator portion mounting. This increased asymmetryresults in a more dominant and well controlled acceleration sensitivityvector which in turn allows greater cancellation of accelerationsensitivity in the composite oscillator. In fact, the tolerance on theasymmetry can be well controlled to give a narrow distribution of theacceleration sensitivity in the dominant axis in terms of both magnitudeand direction, which in turn leads to more effective cancellation ofacceleration sensitivity.

Preferably, each resonator portion includes an elongate member, which,having sides of a relatively large dimension, means that the non-uniformstrain distribution in the x and z directions decays to a sufficientlysmall magnitude so as to enable the active resonance region to bepositioned in a region of uniform stress in relation to these twodirections, thereby reducing the acceleration sensitivity in thesedirections. Similarly the elongate member allows the non-uniform andunbalanced strain distribution in the y direction to be more controlledand predictable over the active resonator region. This increasedmounting asymmetry, resulting from configuring the resonator portion asan elongate member, leads to a yet more dominant and consistentacceleration sensitivity vector than is achievable with, say, resonatorportions configured as circular members, which in turn allows yetgreater cancellation of acceleration sensitivity.

More specifically, elongate cantilever mounted resonators, for examplestrip resonators, tend to have more predictable acceleration sensitivityvectors than those of traditional clip mounted circular resonators. Thismeans that the magnitude of the dominant acceleration sensitivityvectors of individual strip resonators have a narrower distribution, andare therefore more likely to closely match in a composite oscillator,which in turn gives rise to increased acceleration sensitivitycancellation. This further avoids the need for measurement and selectionof matching resonators for use in the composite oscillator.

Preferably, the dominant axes of the resonator portions aresubstantially aligned with the longitudinal axes of the elongateresonator portions, and the acceleration sensitivities of the respectiveactive resonance regions along their dominant axes are directed insubstantially opposite directions. This provides for a higher level ofcancellation of acceleration sensitivity for a given pair of resonatorportions.

The resonator portions may include a piezoelectric crystal or one ormore Micro Electrical Mechanical System (MEMS) devices. Thepiezoelectric crystal may be a Bulk Acoustic Wave (BAW) device or aSurface Acoustic Wave (SAW) device.

In one arrangement, the plurality of resonator portions are electricallycoupled in parallel. An advantage of using a parallel connection ratherthan a series connection is that with a parallel connection, it iseasier to alter (i.e. pull) the resonant frequency of the oscillatorover a larger range (an increase by a factor of approximately two forthe parallel case compared with a decrease by a factor of approximatelytwo for the series case as compared to the individual resonator). Thisis an important parameter as it is desirable to be able to alter theresonant frequency of the oscillator for tuning purposes.

The requirements for two crystals in parallel or series to act as onecrystal in the oscillator requires that their resonant frequencies andequivalent circuit parameters are close, especially for higher Qresonators. This is especially true for parallel connection if theoscillator is operating near series resonance; however if the oscillatoris operating away from series resonance then the requirements are farless demanding. For example for a 10 MHz oscillator operating at 10 pF,i.e. the oscillator sustaining circuit has an input impedance of 10 pF,and typical resonator values of motional capacitance 7 fF and Q of100,000 then a deviation of 100 ppm can be easily tolerated between theresonators. This would still allow the frequency of the compositeoscillator to be pulled +/−100 ppm without encountering problems, whichis significantly higher than that normally required for temperaturecompensation and nominal frequency adjustment, even allowing for ageing.Therefore operating at load allows standard tolerance crystals to beused without additional measurement and selection over and above thatrequired for single resonator oscillators.

In accordance with a second aspect of the present invention, there isprovided a method of manufacturing an oscillator with reducedsensitivity to acceleration, the method comprising the steps ofproviding a plurality of resonator portions, for each said resonatorportion, defining an active resonance region between a set of electrodesso as to be capable of receiving signals at said electrodes caused byoscillator resonance in said active resonance region during operation ofsaid oscillator, mounting each of said resonator portions asymmetricallyin said oscillator using one or more mountings, the asymmetry of saidmountings with respect to each active resonance region defining, foreach active resonance region, a dominant axis passing through the activeresonance region and providing a relatively large accelerationsensitivity along said dominant axis and relatively small accelerationsensitivities along axes orthogonal to said dominant axis, and mountingsaid resonator portions in said oscillator such that the accelerationsensitivities, of the respective active resonance regions along theirrespective dominant axes, are directed in different directions so as toreduce the sensitivity of the oscillator to acceleration.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only, which is made with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows side and plan views of a known resonator and package.

FIG. 1A shows a perspective view of a resonator according to FIG. 1.

FIG. 1B shows a side view of a resonator according to FIG. 1A, subjectedto acceleration along its z axis.

FIG. 1C shows a plan view of a resonator according to FIG. 1A, subjectedto acceleration along its x axis.

FIG. 1D shows a plan view of a resonator according to FIG. 1A, subjectedto acceleration along its y axis.

FIG. 2 is graph showing the acceleration sensitivity vectors of a numberof resonators according to FIG. 1.

FIG. 3 is a plan view of a pair of resonators mounted in a substratesuch that their dominant acceleration sensitivity vectors are directedin substantially opposite directions according to an embodiment of thepresent invention.

FIG. 4 is graph showing the total acceleration sensitivity of a numberof individual and paired resonators according to embodiments of thepresent invention.

FIG. 5 is a circuit schematic of a temperature controlled crystaloscillator with a pair of piezoelectric resonators connected in parallelaccording to an embodiment of the present invention.

FIG. 6 is a schematic of an oscillator with a circuit board where anapplication specific integrated circuit and a pair of resonators aremounted according to an embodiment of the present invention.

FIG. 7 shows side and plan views of an oscillator with a circuit boardand two individual resonators mounted in separate housings according toan embodiment of the present invention.

FIG. 8 is a perspective view of a composite resonator according to asecond embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention sets out to provide an oscillator having reducedsensitivity to acceleration, i.e. an oscillator which exhibits reducedvariation in output resonant frequency when subjected to acceleratingforces.

As described in the background section above, a known resonator andpackage 12 as shown in FIG. 1 containing an asymmetrically mountedresonator 12, exhibits a relatively large and predictable sensitivity toacceleration along its Y axis 8 and relatively small sensitivities toacceleration along the X axis 10 and Z axis 6 which are orthogonal tothe Y axis 8. The Y axis 8 acceleration sensitivity vector dominates theX and Z acceleration sensitivity vectors such that the overall or totalacceleration sensitivity of the oscillator is largely due to thedominant Y axis acceleration sensitivity.

FIG. 2 is a graph 30 showing the acceleration sensitivity vectors of anumber of oscillators according to embodiments of the present invention.The figure shows typical example results for the measured accelerationsensitivity for twenty oscillators, each comprising a strip resonatorrandomly sampled from a production batch, as shown by the left-to-rightscale 34. Here the individual acceleration sensitivity vectors along theX, Y and Z axes have been measured and plotted for each sampleoscillator 34. The individual acceleration sensitivity vectors can bedistinguished in the graph by the use of triangles, circles and diamondsrespectively, as shown legend 36. The top-to-bottom scale 32 is given inparts per billion (10̂⁹) per g (ppb/g), where g is the acceleration dueto gravity, i.e. approximately 9.81 ms⁻². The ppb/g unit is commonlyused for acceleration sensitivity comparisons.

Additionally, the total acceleration sensitivity magnitude |Γ_(total)|,is included in FIG. 2. This is calculated as the square root of the sumof the squares of the individual acceleration sensitivity vectors Γ_(x),Γ_(y), and Γ_(z), along the X, Y and Z axes respectively, according tothe following equation:

|Γ_(total)|=√{square root over (Γ_(x) ²+Γ_(y) ²+Γ_(x) ²)}

FIG. 2 demonstrates that the total acceleration sensitivity magnitude|Γ_(total)|, (plotted with crosses according to legend 36), is dominatedby the individual acceleration sensitivity vector along the Y axisΓ_(y), as the plots for |Γ_(total)| and Γ_(y) match very closely for alltwenty of the sampled oscillators.

FIG. 3 is a plan view 40 of a package including a pair of resonators 42,44 (for example each resonator being similar to that shown in FIG. 1)mounted in a substrate such that their dominant acceleration sensitivityvectors, 46 and 48 respectively, are directed in substantially oppositedirections, i.e. substantially anti-parallel, according to an embodimentof the present invention. The composite oscillator effectively functionsas an average of the two resonators, so a perturbation due to theacceleration sensitivity of one resonator can be cancelled by the otherif the magnitudes are similar and opposite in direction.

FIG. 4 is graph 50 showing the total acceleration sensitivity of anumber of individual and composite oscillators according to embodimentsof the present invention. The composite oscillators include a pair ofpackaged resonators arranged substantially parallel as shown in FIG. 6.

Here the total acceleration sensitivity for each individual resonator isshown by circles and crosses, as shown by legend 56, with thetop-to-bottom scale 52 being the same as for FIG. 2 and theleft-to-right device scale indicating each oscillator. The totalcombined acceleration sensitivity for each composite oscillator pair isalso plotted with pluses, as shown by legend 56.

FIG. 4 shows that for each oscillator, the total accelerationsensitivity is approximately the same for resonator 1 as it is forresonator 2. As the total acceleration sensitivity for each resonator isdominated by the acceleration sensitivity along the Y axis (as shown inFIG. 2), when the individual resonators 1, 2 are combined to form acomposite oscillator and the Y axes are directed in substantiallyopposite directions, these dominant components of the total accelerationsensitivity approximately cancel each other out for each compositeoscillator. This means that the components remaining in the totalacceleration sensitivity for each composite oscillator are approximatelythose along the X and Z axes. As will be appreciated from FIG. 2, theseare of much lower magnitude than the Y axis components, and the totalacceleration of the composite oscillator can be seen to be reduced by afactor between 2.5 and 10.

The above discussion describes the present invention in relation tostrip resonators, which have a cantilever mounted elongate form. A stripresonator is a cut down rectangular version of the more traditionalcircular resonator, which is mounted using two or more clips arranged onopposing edges. Because the active area of strip is in close proximityto the edges, the dimensions of the active area and the resonator needto be precisely designed and controlled to avoid coupling to unwantedresonance modes. This requirement, together with the simplicity of thepackage and cantilever mounting, has lead to highly automated andprecise manufacturing methods. The level of manufacturing control hasthe added advantage of controlling the acceleration sensitivity andmaking it more predictable. Conversely the acceleration sensitivityvectors of a standard clip mounted circular resonator cannot be easilypredicted and must be measured for each individual resonator.

An anti-parallel arrangement of clip mounted circular resonatorsrequires accurate identification of the acceleration sensitivity vectorsand precise alignment of the resonators in anti-parallel in order toreduce the overall effects of sensitivity to acceleration bycancellation. This is not a process which lends itself to high volume,low cost applications and is therefore considered undesirable. Whenusing a cantilever mounted elongated resonator, i.e. a strip resonator,it is easier to achieve alignment because their natural anisotropy helpsto remove the need for the measurement and selection stages.

FIG. 5 is a circuit schematic of a temperature controlled crystaloscillator with a pair of piezoelectric crystal resonators 62, 64connected in parallel according to an embodiment of the presentinvention. Component 60 is a commercially available Application SpecificIntegrated Circuit (ASIC) capable of performing temperature compensationfor the resonators 62, 64, whose performance may vary with temperature.The standard circuit components, component values and signal names aregiven purely for exemplary purposes and should not be taken to limitapplication of the present invention in any form.

Here the resonators 62, 64 are shown to be connected in parallel. Asmentioned above the resonators are preferably electrically connected ina parallel configuration and run at load resonance thereby removing theneed to precisely frequency match the pair of resonators. Despite this,it is not necessary for the resonators to be connected in parallel toact as a composite oscillator and they may also be electricallyconnected in series.

To achieve a high level of acceleration sensitivity cancellation, it isdesirable for two resonators to be placed as near to each other in theoscillator as is practicably possible or convenient. This will reducethe possibility of the two resonators experiencing differentaccelerating forces that may arise in different areas of a housing duefor example to any mechanical flexibility or rotation about an axis nearthe oscillator or such like. FIG. 3 shows one way this could be achievedwhere the resonators are mounted in a cantilever fashion as describedabove on a single substrate, which in turn is mounted in a singlehousing. Where the performance of the oscillator does not require such ahigh degree of reduction in acceleration sensitivity, or where thedesign of the housing satisfactorily eliminates any inhomogeneousacceleration forces therein, then the resonators may be mounted onseparate substrates or indeed in individual housings altogether.

FIG. 6 is a schematic of an oscillator with a circuit board 76, uponwhich are mounted an application specific integrated circuit 70 embeddedin a housing and a pair of resonators 72, 74 according to an embodimentof the present invention. The resonators 72, 74 can be seen to bemounted in an anti-parallel arrangement such that their dominant Y axisacceleration sensitivity vectors are directed in substantially oppositedirections. The ASIC could be attached for example by flip-chip orwire-bond attachment methods.

FIG. 7 shows side 90 and plan 88 views of an oscillator with circuitboard 86 and two individual resonators 82, 84 mounted in separatehousings according to an embodiment of the present invention. In thisembodiment an ASIC 80 is embedded below the two resonators. Analternative to this arrangement would be to use one housing unit forboth resonators.

FIG. 8 is a perspective view of an oscillator according to a secondembodiment of the present invention. In this embodiment, two resonatorportions 106, 108 are integrally formed. The two resonator portions maybe connected via a central portion 102, giving a “see-saw” typearrangement. Each resonator portion 106, 108 has respective mountings110, 112 located in the central portion for mounting the integrallyformed resonator portions 106, 108 to a substrate 114. Each resonatorportion 106, 108 has a set of electrodes, 100, 104 respectively, locatedabove and below each resonator portion which define an active resonanceregion in each resonator portion and allow excitation for sustainingoscillations of the resonators.

In an alternative arrangement, resonator portions 106, 108 could share asingle mounting (not shown). A single mounting for each resonatorportion may serve to reduce mounting stress applied to each of theresonator portions, due to the reduced number of points of contactthrough which mounting stress could be transmitted. The mountingstresses applied to each resonator portion may also be more evenlymatched with a single mounting, which could help to provide more closelymatching, and hence more evenly cancelling acceleration sensitivitiesfor each resonator portion. Further, a single mounting may be simplerand cheaper to manufacture.

The embodiment depicted in FIG. 8 operates on a similar principle toother embodiments of the present invention described above, i.e. due tothe asymmetric nature of the mounting(s), each resonator portion has adominant acceleration sensitivity vector passing through its activeresonance region along the y axis 116; however, due to the centralmounting arrangement, these dominant acceleration sensitivity vectorsare inherently directed in opposing directions. This leads tocancellation of the dominant components, i.e. the components directedalong the y axis 116, in the total acceleration sensitivity for theoscillator in a similar manner to the embodiments described above.

In view of the fact that the strain effect of the mountings decays(approximately exponentially) along the resonator in a direction awayfrom the mountings, exact symmetry of a given resonator portion aboutthe mounting is not required. Indeed, all that is required is for theresonator pairs to share the same centre of symmetry. It will beappreciated that whilst the mounting of the resonator portions into theresonator package remains relatively non-critical, use of more precisemounting arrangements can nevertheless lead to a reduction in the lengthof resonator required to achieve a certain performance specification.

The above embodiments are to be understood as illustrative examples ofthe invention. It is to be understood that any feature described inrelation to any one embodiment may be used alone, or in combination withother features described, and may also be used in combination with oneor more features of any other of the embodiments, or any combination ofany other of the embodiments.

The preferred resonator type for this invention is a BAW (bulk acousticwave) type device, however it is also considered that the same inventionmay be implemented with another type of resonator, for example a SAW(surface acoustic wave) type device.

The above description has focused mainly on piezoelectric typeresonators, but the principles of the present invention could also beapplied to Micro Electrical Mechanical System (MEMS) devices. Eachresonator could be replaced by a MEMS resonator, the operation of whichis known in the art.

The present invention could be applied to oscillators with more than tworesonator portions. For example, three resonator portions could beemployed and their dominant acceleration sensitivity vectors aligned atapproximately 120°.

The above description mentions a cantilever arrangement with twomounting points for mounting resonators in the oscillator, but thepresent invention should not be thus limited. Any type of mountingarrangement with any number of mounting points could be used, theminimum requirement being that some asymmetry is provided which givesrise to a predictable dominant acceleration sensitivity vector for eachresonator. Further any shape of resonator may be employed and need notbe rectangular.

An oscillator according to the present invention could be applied in awide variety of devices where a high purity, stable frequency referenceis required under environments experiencing acceleration or vibration.Such devices could be used in mobile and aerospace applications, butalso devices experiencing vibration from, for example cooling fans infixed installations. Example applications could be telephony, GlobalPositioning System (GPS), radio communications and wireless networking.Such oscillators could be used in many different applications, forexample in military hardware (including land, sea and air) and in theautomotive industry.

The resonator may include a Micro Electrical Mechanical System (MEMS)device.

The principles by which the oscillator of the present inventionfunctions could also be employed in accelerometer devices andarrangements.

Furthermore, equivalents and modifications not described above may alsobe employed without departing from the scope of the invention, which isdefined in the accompanying claims.

1. An oscillator comprising: a plurality of resonator portions, eachsaid resonator portion having an active resonance region defined betweena set of electrodes capable of receiving signals caused by oscillatorresonance in said active resonance region during operation of saidoscillator, each of said resonator portions being mounted in saidoscillator using one or more mountings, each resonator portioncomprising first and second ends, the first end of each resonatorportion being mounted to a substrate via at least one of said one ormore mountings, the second end of each resonator portion being free, andthe resonator portions each having a respective longitudinal axis whichis directed along the resonator portion from the first end to the secondend of the resonator portion, wherein said resonator portions aremounted in said oscillator such that the longitudinal axes of theresonator portions are directed in different directions so as to reducethe sensitivity of the oscillator to acceleration.
 2. An oscillatoraccording to claim 1, wherein said oscillator comprises two or moreindividual resonator portions separately mounted therein.
 3. Anoscillator according to claim 2, wherein two or more of said individualresonator portions are mounted within separate housings.
 4. Anoscillator according to claim 2, wherein two or more of said individualresonator portions are mounted within the same housing.
 5. An oscillatoraccording to claim 1, wherein said oscillator comprises two integrallyformed resonator portions.
 6. An oscillator according to claim 5,wherein said resonator portions are connected via a central portion andsaid mountings for each resonator portion are located in said centralportion.
 7. An oscillator according to claim 1, wherein said one or moremountings comprise a cantilever mounting arrangement.
 8. An oscillatoraccording to claim 7, wherein said cantilever mounting arrangementcomprises a pair of mountings adjacent the first end of each resonatorportion, the connections providing both mechanical coupling to thesubstrate and electrical coupling to said set of electrodes.
 9. Anoscillator according to claim 1, wherein each active resonance region islocated towards the second end of its respective resonator portion. 10.An oscillator according to claim 1, wherein each said resonator portioncomprises an elongate member having sides of a relatively largedimension and top and bottom ends having a relatively small dimensionand a longitudinal axis extending between said top and bottom ends. 11.An oscillator according to claim 10, wherein said mounted and free endsof each elongate resonator portion correspond to the top and bottom endsof each elongate resonator portion.
 12. An oscillator according to claim10, wherein each said active region has a relatively large accelerationsensitivity along a dominant axis, and a relatively small accelerationsensitivity along axes orthogonal to said dominant axis, and whereinsaid dominant axes are substantially aligned with the longitudinal axesof said elongate resonator portions.
 13. An oscillator according toclaim 1, wherein the acceleration sensitivities of the respective activeresonance regions are directed in substantially opposite directions. 14.An oscillator according to claim 1, wherein said resonator portionscomprise a piezoelectric crystal.
 15. An oscillator according to claim14, wherein one or more of said resonator portions comprises a BulkAcoustic Wave (BAW) device.
 16. An oscillator according to claim 14,wherein one or more of said resonator portions comprises a SurfaceAcoustic Wave (SAW) device.
 17. An oscillator according to claim 14,wherein one or more resonator portion comprises one or more of: quartz(SiO2), lithium tantalate (LiTaO3), lithium niobate (LiNbO3).
 18. Anoscillator according to claim 1, wherein said resonator portioncomprises one or more Micro Electrical Mechanical System (MEMS) devices.19. An oscillator according to claim 1, wherein said plurality ofresonator portions are electrically coupled in series.
 20. An oscillatoraccording to claim 1, wherein said plurality of resonator portions areelectrically coupled in parallel.
 21. An oscillator circuit comprisingan oscillator according to claim 1 and further comprising an integratedcircuit electrically coupled to each of the resonator portions forproviding a frequency output that exhibits substantially reducedsensitivity to acceleration.
 22. A mobile telephone comprising anoscillator according to claim
 1. 23. A mobile global positioning system(GPS) device comprising an oscillator according to claim
 1. 24. A methodof manufacturing an oscillator with reduced sensitivity to acceleration,the method comprising the steps of: providing a plurality of resonatorportions; for each said resonator portion, defining an active resonanceregion between a set of electrodes so as to be capable of receivingsignals at said electrodes caused by oscillator resonance in said activeresonance region during operation of said oscillator; mounting each ofsaid resonator portions in said oscillator using one or more mountings,each resonator portion comprising first and second ends, the first endof each resonator portion being mounted to a substrate via at least oneof said one or more mountings, the second end of each resonator portionbeing free, the resonator portions each having a respective longitudinalaxis which is directed along the resonator portion from the first end tothe second end of the resonator portion; and mounting said resonatorportions in said oscillator such that the longitudinal axes of theresonator portions are directed in different directions so as to reducethe sensitivity of the oscillator to acceleration.